However, concerns have been raised in the UK about the exaggeration of some of the discounts on offer and a time limit to take up services.

In one case, the Advertising Standards Authority said it had found that there was no evidence that advertised deals were available. This included deals claiming that consumers could buy a bouquet of flowers for £8 and that they could take advantage of an "all you can eat in London for £3" offer.

Agreed changes

Groupon has agreed to make sure that prices in its adverts are accurate and honest and that any limitations on availability are made clear.

Its undertakings include:

Ensuring that the advertised discount is accurate, honest and transparent

Conducting an accurate, honest and realistic assessment that a trader can offer the goods and services in the quantity and timeframe suggested

Clearly outlining any limitations on deals

Adequately substantiating any health and beauty product claims

Ensuring terms and conditions are fair

Applying the rules on refunds and cancellation rights for anyone buying online

The OFT warned Groupon that it would monitor complaints closely and that any breach of the undertakings could lead to court enforcement orders.

"Collective buying and discount schemes can offer real benefits for both consumers and merchants," said Cavendish Elithorn, from the OFT.

It is like any shopping on the internet - you should shop aroundKaren Bellerby, Groupon customer

"The market is growing rapidly, but it is important that consumers benefit from consumer protection law as well as from the discounted offers.

"Groupon has cooperated fully with our investigation and is making changes to its business practices to address our concerns. We will be monitoring the situation closely to ensure that consumers benefit from these improvements."

'Shop around'

Groupon customer Sarah Luty, a 31-year-old from Luton, said she had become much more selective on which deals to buy after finding it difficult to redeem one voucher.

"I bought a spa day from Groupon which included a massage, glass of bubbly and full use of the facilities," she said.

"When I bought it, they had sold so many I had to book a midweek slot five months later, as that was all that was available. Meanwhile in those five months, Groupon continued to sell spa days to the same place."

Another customer - Karen Bellerby - told the BBC News website that Groupon had some good deals, but it was important for customers not to blindly accept everything was cheap.

The charity worker from Colchester said that one of the best deals she had bought was having her hair done for £29, but the electronic goods were often no cheaper than elsewhere, and there were some restrictions on travel deals.

"Sometimes you might get a similar deal by going direct. It is like any shopping on the internet - you should shop around," she said.

"We have independently made many improvements since early 2011 and have worked transparently and constructively with the OFT to identify areas that require further changes. We take their concerns very seriously and will be willingly implementing the recommended changes."

In February, its first set of results since listing on Nasdaq stock exchange in November, Groupon reported a net loss of $42.7m (£27.0m).